Electroluminescent devices

ABSTRACT

An electroluminescent device comprising: a first charge carrier injecting layer for injecting positive charge carriers; a second charge carrier injecting layer for injecting negative charge carriers; and a light-emissive layer located between the charge carrier injecting layers and comprising a mixture of: a first component for accepting positive charge carriers from the first charge carrier injecting layer; a second component for accepting negative charge carriers from the second charge carrier injecting layer; and a third, organic light-emissive component for generating light as a result of combination of charge carriers from the first and second components; at least one of the first, second and third components forming a type II semiconductor interface with another of the first, second and third components.

RELATED APPLICATIONS

This is a continuation of U.S. Ser. No. 11/398,258, filed Apr. 5, 2006(now U.S. Pat. No. 7,449,714), which is a continuation of U.S. Ser. No.10/682,204, filed Oct. 10, 2003, (now U.S. Pat. No. 7,078,251), which isa continuation of U.S. Ser. No. 09/508,367 filed Jan. 3, 2002 (now U.S.Pat. No. 6,897,473), which is a 371 of PCT/GB99/00741, filed Mar. 12,1999, which claims the benefit of priority of GB 9805476.0, filed Mar.13, 1998, all of which are incorporated herein by reference in theirentirety.

FIELD OF THE INVENTION

This invention relates to electroluminescent devices, especially thosethat employ an organic material for light emission.

BACKGROUND OF THE INVENTION

Electroluminescent devices that employ an organic material for lightemission are described in PCT/WO90/13148 and U.S. Pat. No. 4,539,507,the contents of both of which are incorporated herein by reference. Thebasic structure of these devices is a light-emissive organic layer, forinstance a film of a poly(p-phenylenevinylene (“PPV”), sandwichedbetween two electrodes. One of the electrodes (the cathode) injectsnegative charge carriers (electrons) and the other electrode (the anode)injects positive charge carriers (holes). The electrons and holescombine in the organic layer generating photons. In PCT/WO90/13148 theorganic light-emissive material is a polymer. In U.S. Pat. No. 4,539,507the organic light-emissive material is of the class known as smallmolecule materials, such as (8-hydroxyquinolino)aluminium (“Alq”). In apractical device, one of the electrodes is typically transparent, toallow the photons to escape the device.

These devices have great potential for displays. However, there areseveral significant problems. One is to make the device efficient,particularly as measured by its power efficiency and its externalefficiency. Another is to reduce the voltage at which peak efficiency isobtained.

As a preliminary point, it should be noted that the values stated herefor energy levels, workfunctions etc. are generally illustrative ratherthan absolute. The workfunction of ITO can vary widely. Numbers quotedin the literature suggest a range between 4 and 5.2 eV. The 4.8 eV valueused here serves as an illustrative rather than an absolute value. Theapplicant has carried out Kelvin probe measurements which suggest that4.8 eV is a reasonable value. However, it is well known that the actualvalue can depend on ITO deposition process and history. For organicsemiconductors important characteristics are the binding energies,measured with respect to the vacuum level of the electronic energylevels, particularly the “highest occupied molecular orbital” (“HOMO”)and “lowest unoccupied molecular orbital” (“LUMO”) levels. These can beestimated from measurements of photoemission and particularlymeasurements of the electrochemical potentials for oxidation andreduction. It is well understood in the field that such energies areaffected by a number of factors, such as the local environment near aninterface, and is the point on the curve (peak) from which the value isdetermined—e.g. peak, peak base, half-way point—so the use of suchvalues is indicative rather than quantitative. However, the relativevalues are significant.

FIG. 1 a shows a cross section of a typical device for emitting greenlight. FIG. 1 b shows the energy levels across the device. The anode 1is a layer of transparent indium-tin oxide (“ITO”) with a workfunctionof 4.8 eV. The cathode 2 is a LiAl layer of with a workfunction of 2.4eV. Between the electrodes is a light-emissive layer 3 of PPV, having aLUMO energy level 5 at around 2.7 eV and a HOMO energy level 6 at around5.2 eV. Holes and electrons that are injected into the device recombineradiatively in the PPV layer. A helpful but not essential feature of thedevice is the hole transport layer 4 of doped polyethylenedioxythiophene (“PEDOT”) (see EP 0 686 662 and Bayer AG's ProvisionalProduct Information Sheet for Trial Product Al 4071). This provides anintermediate energy level at 4.8 eV, which helps the holes injected fromthe ITO to reach the HOMO level in the PPV.

Other organic light-emissive materials, having different optical gaps,can take the place of the PPV in order to generate light of othercolours. However, at larger optical gaps, towards the blue end of thevisible spectrum, the HOMO level is generally well below thecorresponding energy level of the TO. This makes it difficult to injectholes into the emissive layer, i.e. high electric fields are required inorder to encourage holes to inject into the semiconductor layer. Onesolution to this problem would be to choose another material for theanode, but it is difficult to find a preferable alternative because ITOhas good transparency, low sheet resistance and established processingroutes. Another solution is to add further hole transport layers, so asto provide a series of intermediate energy steps between the anode andthe emissive layer. However, where the layers are deposited fromsolution it is difficult to avoid one layer being disrupted when thenext is deposited, and problems can arise with voids or material trappedbetween the increased number of inter-layer boundaries.

Considerable advantages can be had from using a plurality of organicsemiconductors within a diode structure; critical to the functioning ofsuch structures is the nature of the interface electronic structurebetween any two components in contact with one another. A commonstarting point for such descriptions is that well-known forheterojunctions formed in epitaxially-grown III-V semiconductors.Heterojunctions are classified into classes which include: type I, inwhich the LUMO and HOMO levels of one material (material A) lie withinthe LUMO-HOMO energy gap of the second material (material B), asillustrated in FIG. 2 a, and type II, in which the minimum energydifference between the highest HOMO state and the lowest LUMO state isbetween levels on different sides of the heterojunction, as illustratedin FIG. 2 b. It is generally considered that an electron-hole pair thatis in the immediate vicinity of such heterojunctions will arrange sothat the electron occupies the lowest LUMO level, and the hole occupiesthe highest HOMO level. Thus, the electron and hole are present on thesame side of the junction for a type I heterojunction, but are separatedfor the type II heterojunction. An important consequence of this is thatelectron-hole capture and subsequent light emission is expected for typeI but not for type II heterojunctions.

There have been some attempts to combine components in blue-emissivelayers. In “Highly Efficient Blue Electroluminescence from aDistyrylarylene Emitting Layer with a new Dopant”, Hosokawa et al.,Appl. Phys. Lett. 67 (26), 25 Dec. 1995, pp 3853-5 a small moleculedevice has an emissive layer in which DPVBi is blended with BCzVB orBczVBi. The dopants have a slightly smaller bandgap and a displaced HOMOposition compared to the host material. The observed light emission isonly from the dopant. This is explained by the authors as arising fromFörster energy transfer due to the smaller energy of an exciton on thedopant molecules. “Efficient Blue-Light Emitting Devices from ConjugatedPolymer Blends”, Birgerson et al., Adv. Mater. 1996, 8, No. 12, pp 982-5describes a blue-light emitting device which employs conjugated polymerblends. The emissive layer of the device consists of a blend of PDHPTwith PDPP. These materials form a type I semiconductor interface (seeFIG. 2 a), so light emission is from the PDHPT alone. The paperemphasises that “it is a necessary but not sufficient requirement thatthe HOMO-LUMO gap of the light-emitting (guest) polymer be smaller thanthat of the host polymer. An additional condition is that . . . the HOMOenergy level of the guest polymer must be at a lower binding energy thanthat of the host polymer, and the LUMO energy level of the guest polymermust be at a higher binding energy than that of the host polymer”. Otherdevices having type I interfaces at the emissive layer are described inEP 0 532 798 A1 (Mori et al.) and U.S. Pat. No. 5,378,519 (Kikuchi etal.).

Two-layer EL devices which exploit the high electron affinity ofcyano-derivatives of PPV have shown high efficiencies, as described inU.S. Pat. No. 5,514,878. However, when mixtures are formed with CN-PPVand the soluble PPV, MEH-PPV, as described in “Efficient Photodiodesfrom Interpenetrating Polymer Networks”, J J M Halls et al., Nature,Vol. 376, 10 August 1995, pp 498-500 and U.S. Pat. No. 5,670,791, strongquenching of luminescence is observed.

“Oxadiazole-Containing Conjugated Polymers for Light-Emitting Diodes”,Peng et al., Adv. Mater. 1998, 10, No. 9 describes a light-emittingdevice in which the emissive layer comprises an oxadiazole-containingPPV polymer. The oxadiazole is present to aid electron transport. It isnoted that “the PPV segment can function as both the hole transporterand the emitter”.

In “Efficient Blue LEDs from a Partially Conjugated Si-Containing PPVCopolymer in a Double-Layer Configuration”, Garten et al., Adv. Mater.,1997, 9, No. 2, pp 127-131 a light-emissive device has an emissive layerin which Si-PPV is diluted with PVK to reduce aggregation. Thephotoluminescent efficiency of the device is observed to increase whenaggregation is reduced.

“Blue Light-Emitting Devices Based on Novel Polymer Blends”, Cimrová etal., Adv. Mater. 1998, 10, No. 9 describes light-emitting devices whoseemitting layers comprise a blend of two polymers having “almostidentical HOMO levels”.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is providedan electroluminescent device comprising: a first charge carrierinjecting layer for injecting positive charge carriers; a second chargecarrier injecting layer for injecting negative charge carriers; and alight-emissive layer located between the charge carrier injecting layersand comprising a mixture of: a first component for accepting positivecharge carriers from the first charge carrier injecting layer; a secondcomponent for accepting negative charge carriers from the second chargecarrier injecting layer; and a third, organic light-emissive componentfor generating light as a result of combination of charge carriers fromthe first and second components; at least one of the first, second andthird components forming a type II semiconductor interface with anotherof the first, second and third components.

The process of accepting and combining charge carriers may includeacceptance of an exciton from another component and/or acceptance ofseparate positive and negative charge carriers which subsequently forman exciton.

Preferably one or all of the said components of the light-emissive layeris/are phase separated to some extent (e.g. partially or fully) in thelight-emissive layer. The light-emissive layer suitably comprisesregions of each of the said components, which are preferably dispersedthrough the light-emissive layer. Each of those regions suitablycomprises substantially only one of the said components, and preferablyhas the electronic characteristics provided by that component. Thecomponents may be evenly or unevenly distributed in the light-emissivelayer. One or more of the components may be concentrated near theinterfaces of the light-emissive layer with the first or second chargecarrier injecting layer. The concentration may be such that near thatinterface that component is substantially undiluted by another componentof the mixture. Thus, that component may suitably approach or reach fullconcentration at that interface. It is preferred that near the interfacewith the first charge carrier injecting layer there is a greaterconcentration of the first component than in the central region of thelight-emissive layer and/or near the interface with the second chargecarrier injecting layer. It is preferred that near the interface withthe second charge carrier injecting layer there is a greaterconcentration of the second component than in the central region of thelight-emissive layer and/or near the interface with the first chargecarrier injecting layer. The concentration of the first component in thelight-emissive layer may increase towards the first charge carrierinjecting layer. The concentration of the second component in thelight-emissive layer may increase towards the second charge carrierinjecting layer.

The layer on to which the light-emissive layer is deposited may betreated in order to influence the said concentrations. If thelight-emissive layer is deposited directly or indirectly on to the firstinjecting layer then the first layer may be treated to encourage agreater concentration of the first component near it. If thelight-emissive layer is deposited directly or indirectly on to thesecond injecting layer then the second layer may be treated to encouragea greater concentration of the second component near it. The treatmentcould, for example, be surface modification (e.g. the application of anoxygen plasma) or depositing of another layer of a material, for examplea material for which the first component has a greater affinity thandoes the second component or for which the second component has agreater affinity than does the first component. That material may be orcomprise the first component or the second component. The surfacemodification suitably affects the surface free energy of the surface onto which the light-emissive layer is to be deposited.

Two or more of the components of the emissive layer may be provided asfunctional chemical units or moieties of a single molecule. Any furthercomponents of the layer may be provided by one or more further moleculesphysically mixed with the said single molecule. Alternatively, all thecomponents of the emissive layer may be provided by respective differentmolecules physically mixed together. Where a single molecule providesmore than one component those components could be combined as acopolymer (e.g. in main chain, side chain, block or random form). One ormore of the components could be provided as a pendant group of a polymerchain of another one or more of the components. Where a single moleculeprovides more than one component the components provided by thatmolecule preferably include the third component. Suitably the thirdcomponent and at least one of the first and second components areprovided as a copolymer. Suitably the third component is provided as apendant group of a polymer chain of the first and/or second components.Suitably the first and/or second components are provided as one or morependant groups of a polymer chain of the third component.

The light-emissive layer is preferably formed by deposition of thefirst, second and third components together. Preferably all thecomponents of the emissive layer are soluble, and most preferably allare soluble in the same solvent or in the same solvent mixture. This maypermit the components to be conveniently co-deposited from solution. Thelight-emissive layer may comprise two or more sub-layers each comprisingthe first, second and third components.

Preferably one or more of the first, second and third components forms atype II semiconductor interface with another of the first, second andthird components. A distinction can be made between type II interfaceswhich do not lead to charge separation (which may be referred to a“luminescent type II interfaces”) and those that do lead to chargeseparation and which, by this or another mechanism, tend to quenchluminescence (“non-luminescent type II interfaces”). The type IIinterfaces referred to herein are suitably of the luminescent type.Luminescent and non-luminescent interfaces can easily be characterisedby forming suitable interfaces (as bi-layers or as mixtures formed fromsolution) and measuring their luminescence behaviour under opticalexcitation. Methods for measuring the absolute luminescence efficiencyare referred to in the paper by Halls et al. cited above.

The applicant considers that it might be possible to understand theunderlying principles which govern the behaviour of such type IIinterfaces by taking into account the role of the binding energy betweenelectron and hole when formed as the neutral excited electronic state(exciton). This “exciton binding energy” is in part due to theelectrostatic attraction between electron and hole, and seems to be muchstronger in the molecular and polymeric semiconductors—which arepreferred for use in embodiments of the present invention as providingone or more of the first, second and third components—than in inorganicsemiconductors such as III-V materials. The exciton binding energy mayact to keep both electron and hole on the same side of a heterojunction.Therefore, in order to achieve charge separation, the energy offsets atthe heterojunction as illustrated for the type II case in FIG. 2 b)between HOMO and LUMO (as appropriate) levels may be preferred to begreater than the exciton binding energy.

Most preferably all of the first, second and third components form typeII semiconductor interfaces with the others of the first, second andthird components. The first component may form a type II semiconductorinterface with the second component. The second component may form atype II semiconductor interface with the third component. The firstcomponent may form a type II semiconductor interface with the thirdcomponent. As explained above, any or all these are suitably“luminescent type II interfaces”. One potential physical structure ofsuch an interface may be that it is a type II interface in which it ismore energetically favourable for an exciton to form on one of thecomponents (preferably but not necessarily a luminescent component) thanfor an electron/hole pair to dissociate onto separate components. Thiswill normally be due to there being a relatively small offset betweenthe HOMO and/or LUMO levels of components of the device. In manypreferred practical situations this will be due to the energy gapbetween the HOMO level of one component and that of another componenthaving a lower HOMO level less than the binding energy of an exciton onthat said one component. In such a system electrons may be injected tothe lower HOMO level of the said other component and holes to the higherLUMO level of the said one component. However, it can be energeticallyefficient for the electron and hole to combine to form an exciton on thesaid one component. Thus advantage may be had from efficient injectionat the said lower HOMO and higher LUMO levels (allowing a greater choiceof materials for electrodes of device) whilst maintaining emission fromthe device. The applicant has found that this effect can lead to aremarkable increase in the efficiency of a light emissive device.

The light-emissive layer may comprise other materials, or may consist(or essentially consist) of the first, second and third components,together optionally with any impurities.

The third component is preferably a material that is emissive in thevisible (e.g. the red, green or blue) and/or near infrared and/or nearultraviolet regions of the spectrum. The optical gap of the thirdcomponent is preferably greater than 1.8 eV. When the device is in usethere is preferably no (or substantially no) emission from the first andsecond components. The third component may suitably have an optical gapsmaller than the optical gaps of the first and second components. The“optical gap” of a material may be measured as the photon energy atwhich the material exhibits strong optical absorption. The thirdmaterial is preferably a highly efficient luminescent material.

In some circumstances it may be advantageous for the first component tohave a LUMO energy level between the LUMO energy levels of the secondand third components, suitably to assist the movement of negative chargecarriers between the second and third components. The first componentsuitably has a HOMO energy level between the HOMO energy levels of thesecond and third components, suitably to assist the movement of positivecharge carriers to the second and/or the third components. The firstcomponent suitably has a HOMO energy level greater than or equal to theenergy level of the first charge injecting layer.

The first, second and third components may each be an organic material,suitably a polymer, preferably a conjugated or partially conjugatedpolymer. Suitable materials include PPV,poly(2-methoxy-5(2′-ethyl)hexyloxyphenylene-vinylene) (“MEH-PPV”), aPPV-derivative (e.g. a di-alkoxy or di-alkyl derivative), a polyfluoreneand/or a co-polymer incorporating polyfluorene segments, PPVs and/orrelated co-polymers. The first and second components (in addition to thethird component) may be of light-emissive materials. The first componentis suitably a conjugated polymer capable of accepting positive chargecarriers from the first charge carrier injecting layer and containingamine groups in the main chain and/or as pendant groups. Alternativematerials include organic molecular light-emitting materials, e.g. Alq₃,or any other small sublimed molecule or conjugated polymerelectroluminescent material as known in the prior art. The firstcomponent may bepoly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-secbutylphenyl)imino)-1,4-phenylene))(“TFB”). The second component may be poly(2,7-(9,9-di-n-octylfluorene)(“F8”). The third component may bepoly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-methylphenyl)imino)-1,4-phenylene-((4-methylphenyl)imino)-1,4-phenylene))(“PFM”), poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-methoxyphenyl)imino)-1,4-phenylene-((4-methoxyphenyl)imino)-1,4-phenylene))(“PFMO”) or poly(2,7-(9,9-di-n-octylfluorene)-3,6-Benzothiadiazole)(“F8BT”). (See FIG. 3). The third component may be a soluble PPV. Othermaterials could be used.

The first charge carrier injecting layer may be a positive chargecarrier transport layer which is located between the light-emissivelayer and an anode electrode layer, or may be an anode electrode layer.The second charge carrier injecting layer may be a negative chargecarrier transport layer which is located between the light-emissivelayer and a cathode electrode layer, or may be a cathode electrodelayer. Any electrode layer and/or charge transport layer is suitablylight transmissive, and preferably transparent, suitably at thefrequency of light emission from the device.

The anode electrode layer suitably has a workfunction greater than 4.0eV. The cathode electrode layer suitably has a workfunction less than3.5 eV.

According to a second aspect of the present invention there is providedan electroluminescent device comprising: a first charge carrierinjecting layer for injecting positive charge carriers; a second chargecarrier injecting layer for injecting negative charge carriers; and alight-emissive layer located between the charge carrier injecting layersand comprising a mixture of: a first organic light-emissive componentfor accepting and combining positive charge carriers from the firstcharge carrier injecting layer and negative charge carriers from thesecond light-emissive component to generate light; a second organiclight-emissive component for accepting and combining negative chargecarriers from the second charge carrier injecting layer and positivecharge carriers from the first light-emissive component to generatelight; the first and second components forming a type II semiconductorinterface with each other.

The said type II semiconductor interface is preferably a luminescenttype II interface.

The said components of the light-emissive layer may be phase separatedto some extent (e.g. partially or fully) in the light-emissive layer.The light-emissive layer suitably comprises regions of each of the saidcomponents, which are preferably dispersed through the light-emissivelayer. Each of those regions suitably comprises substantially only oneof the said components, and preferably has the electroniccharacteristics provided by that component. The components may be evenlyor unevenly distributed in the light-emissive layer. One of thecomponents may be concentrated near one or both of the interfaces of thelight-emissive layer with the first or second charge carrier injectinglayer. That component is preferably the first component. Theconcentration of that component may be such that near one or both ofthose interfaces it is substantially undiluted by the other component ofthe mixture. Thus, that component may suitably approach or reach fullconcentration at one or both of those that interfaces.

The components of the emissive layer may be provided as functionalchemical units or moieties of a single molecule, or as a physicallymixture or different molecules. Where a single molecule provides morethan one component those components could be combined as a copolymer(e.g. in main chain, side chain, block or random form). One of thecomponents (either the first component or the second component) could beprovided as a pendant group of a polymer chain of the other component(ether the second component or the first component).

Preferably both the components of the emissive layer are soluble, andmost preferably both are soluble in the same solvent or in the samesolvent mixture. This may permit the components to be convenientlyco-deposited from solution.

The first component suitably has a HOMO energy level above that of thesecond component. The second component suitably has a LUMO energy levelabove that of the third component.

The light-emissive layer may comprise other materials, or may consist(or essentially consist) of the first and second components, togetheroptionally with any impurities.

Either or both of the first and second components are preferablymaterials that are emissive in the visible (e.g. the red, green or blue)and/or near infrared and/or near ultraviolet regions of the spectrum.The optical gap of either or both of the first and second components ispreferably greater than 1.8 eV.

The first and second components may each be an organic material,suitably a polymer, preferably a conjugated or partially conjugatedpolymer. Suitable materials include PPV,poly(2-methoxy-5(2′-ethyl)hexyloxyphenylene-vinylene) (“MEH-PPV”), aPPV-derivative (e.g. a di-alkoxy or di-alkyl derivative), a polyfluoreneand/or a co-polymer incorporating polyfluorene segments, PPVs and/orrelated co-polymers. Alternative materials include organic molecularlight-emitting materials, e.g. Alq₃, or any other small sublimedmolecule or conjugated polymer electroluminescent material as known inthe prior art. The second component may, for example, be F8 or aporphyrin. The first component may, for example, be TFB. The firstcomponent could be a conjugated polymer capable of accepting positivecharge carriers from the first charge carrier injecting layer andcontaining amine groups in the main chain and/or as pendant groups.Other materials could be used.

The first charge carrier injecting layer may be a positive chargecarrier transport layer which is located between the light-emissivelayer and an anode electrode layer, or may be an anode electrode layer.The second charge carrier injecting layer may be a negative chargecarrier transport layer which is located between the light-emissivelayer and a cathode electrode layer, or may be a cathode electrodelayer. Any electrode layer and/or charge transport layer is suitablylight transmissive, and preferably transparent, suitably at thefrequency of light emission from the device.

The anode electrode layer suitably has a workfunction greater than 4.0eV. The cathode electrode layer suitably has a workfunction less than3.0 eV.

According to a third aspect of the present invention there is providedan electroluminescent device comprising: a first charge carrierinjecting layer for injecting positive charge carriers; a second chargecarrier injecting layer for injecting negative charge carriers; anorganic light-emissive layer located between the charge carrierinjecting layers; and an organic charge transport layer located betweenthe light-emissive layer and one of the charge carrier injecting layers,wherein the heterojunction formed between the transport layer and thelight-emissive layer is a luminescent type II heterojunction.

According to a fourth aspect of the present invention there is provideda method for forming an electroluminescent device, comprising:depositing a first charge carrier injecting layer for injecting chargecarriers of a first polarity; depositing a light-emissive layer over thefirst charge carrier injecting layer, the light-emissive layercomprising a mixture of: a first component for accepting charge carriersfrom the first charge carrier injecting layer; a second component foraccepting charge carriers of the opposite polarity from a second chargecarrier injecting layer; and a third, organic light-emissive componentfor generating light as a result of combination of charge carriers fromthe first and second components; at least one of the first, second andthird components forming a type II semiconductor interface with anotherof the first, second and third components; and depositing the secondcharge carrier injecting layer over the light-emissive layer forinjecting charge carriers of the said opposite polarity.

According to a fifth aspect of the present invention there is provided amethod for forming an electroluminescent device, comprising: depositinga first charge carrier injecting layer for injecting charge carriers ofa first polarity; depositing a light-emissive layer comprising a mixtureof: a first organic light-emissive component for accepting and combiningcharge carriers of the first polarity from the first charge carrierinjecting layer and charge carriers of the other polarity from a secondlight-emissive component to generate light; and the second organiclight-emissive component for accepting and combining charge carriers ofthe said opposite polarity from the second charge carrier injectinglayer and charge carriers of the first polarity from the firstlight-emissive component to generate light; the first and secondcomponents forming a type II semiconductor interface with each other;and depositing the second charge carrier injecting layer for injectingcharge carriers of the said opposite polarity.

In the fourth and fifth aspects of the invention the first, second and(where present) third components are preferably deposited together. Thesaid first polarity is preferably positive, but could be negative. Thesaid opposite polarity is preferably negative, but could be positive.The said methods preferably comprise the step of treating the firstcharge carrier injecting layer prior to deposition of the light-emissivelayer to influence the phase structure of the light-emissive layer. Thismay be to encourage a greater concentration of the first component nearthe first charge carrier injecting layer.

For all aspects of the present invention it is preferred that theapplied voltage at which the device has maximum power efficiency orexternal efficiency is below 10 V, preferably below 7 V and mostpreferably below 4 V. For both aspects of the invention it is preferredthat the device has a peak power efficiency equivalent to devicesemitting in the green of greater than 1 lm/W, preferably greater than 2lm/W and most preferably greater than 6 lm/W. For both aspects of theinvention it is preferred that the device has a peak external efficiencyequivalent to devices emitting in the green of greater than 2 Cd/A,preferably greater than 5 Cd/A and preferably greater than 7 Cd/A.

For all aspects of the present invention it is preferred that thethickness of the emissive layer is below 400 nm, and most preferably inthe range from 40 to 160 nm.

An aspect important to appreciation of the present invention, as furtherdeveloped below, is that the conventional description of heterojunctionsdoes not generally apply for the molecular and polymeric semiconductorswhich are described here. In particular, although type IIheterojunctions are sometimes effective at charge separation (with nolight emission), it has been found that there are importantcircumstances when they are capable of providing efficient emission oflight. As illustrated below, this possibility for the desirableproperties of a “type II” heterojunction is not recognised in the priorart known to the applicant. Furthermore, there is no prior art known tothe applicant which leads to the realisation of EL diodes which containtype II junctions and which shows reduced operating voltages by virtueof the presence of the several semiconductor components present.

A device according to the present invention may have a photoluminescenceefficiency that is not substantially less than the photoluminescenceefficiency of the emissive component of the emissive layer (e.g. thethird component) in unblended form. That photoluminescence efficiencymay suitably greater than 30%.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example withreference to the accompanying drawings, in which:

FIG. 1 a shows a cross section of a typical electroluminescent devicefor emitting green light;

FIG. 1 b shows the energy levels across the device of FIG. 1 a;

FIG. 2 a is an energy band diagram for a heterojunction of type I, inwhich the LUMO and HOMO levels of one material lie within the LUMO-HOMOenergy gap of the second material;

FIG. 2 b is an energy band diagram for a heterojunction of type II, inwhich the minimum energy difference between the highest HOMO state andthe lowest LUMO state is between levels on different sides of theheterojunction;

FIG. 3 shows the chemical structures of some materials discussed below;

FIG. 4 is a band diagram for an electroluminescent device;

FIG. 5 is a graph plotting the efficiency of the device of FIG. 4against voltage;

FIG. 6 is a band diagram for a second electroluminescent device;

FIG. 7 is a graph plotting the efficiency of the device of FIG. 6against voltage;

FIG. 8 shows the emission spectrum of the device of FIG. 6;

FIG. 9 is a band diagram for a third electroluminescent device;

FIG. 10 is a graph plotting the efficiency of the device of FIG. 9against voltage;

FIG. 11 shows the emission spectrum of the device of FIG. 9;

FIG. 12 is a band diagram for a fourth electroluminescent device;

FIG. 13 is a graph plotting the efficiency of the device of FIG. 12against voltage;

FIG. 14 shows the emission spectrum of the device of FIG. 12;

FIG. 15 is a band diagram for a fifth electroluminescent device;

FIG. 16 is a graph plotting the efficiency of the device of FIG. 15against voltage;

FIG. 17 is a graph plotting the efficiency of a sixth electroluminescentdevice against voltage;

FIG. 18 is a band diagram for the device of FIG. 17;

FIG. 19 is a graph plotting the efficiency of a seventhelectroluminescent device against voltage;

FIG. 20 is a band diagram for an eighth electroluminescent device;

FIG. 21 is a band diagram for a ninth electroluminescent device;

FIG. 22 plots the characteristics of two further electroluminescentdevices;

FIG. 23 plots the characteristics of an 8 pixel electroluminescentdevice;

FIG. 24 plots the characteristics of another 8 pixel electroluminescentdevice;

FIG. 25 plots the characteristics of a set of devices having from 0% to20% TFB;

FIG. 26 plots the characteristics of a 4 pixel electroluminescent devicewhose emissive layers comprise a TPD polymer;

FIG. 27 shows the structure of Bis-DMOS PPV;

FIG. 28 shows the characteristics of two further devices; and

FIG. 29 shows the results of cyclic voltammetry analysis of a TPDside-chain polymer.

DETAILED DESCRIPTION

Table 1 gives material properties of some light-emissive materials:

TABLE 1 % PL % PL HOMO LUMO Optical Emission Material Efficiency ¹Efficiency ² Level (eV) Level (eV) gap (eV) Colour F8 80 50 5.8 2.8 3.0Blue F8BT 80 5.9 3.3 2.4 Green TFB 40 15 5.3 2.3 3.0 Blue PFMO 40 13 5.02.0 3.0 Blue PFM 20 5 5.0 2.1 2.9 Blue 5F8BT 95 55 5.9 3.5 2.4 GreenBis-DMOS 5.7 3.5 2.2 Green PPV PPV A ³ 5.6 3.2 2.6 Green Notes to table1: ¹ Photoluminescence (PL) efficiencies measured using the technique ofHalls et al. (see above). ² Measured using a refined technique based onthat of Halls et al. ³ See discussion of FIG. 25 below.

The HOMO positions were estimated from electrochemical measurement. Theoptical gaps were determined from the UV/visible absorbance spectrum.The LUMO positions were estimated from the HOMO position and the opticalgap. 5F8BT is an abbreviation for a blend of 5% F8BT with 95% F8 w/w.

Of the blue emitters, F8 has the highest PL efficiency of thesematerials. Therefore, of these materials it would normally be thematerial of choice for a blue emissive layer. FIG. 4 is a band diagramfor a device in which the emissive layer is F8. A layer ofpoly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-(4-imino(benzoicacid))-1,4-phenylene-(4-imino(benzoic acid))-1,4-phenylene)) (“BFA”) isincluded as an intermediate hole transport layer. Alternatives for BFAare PEDOT-PSS (polyethylene dioxythiophene doped withpolystyrene-sulphonate to modify its conductivity—available from BayerAG and described in UK patent application number 9703172.8),polyaniline, PVK etc. The hole transport layer also serves to block thepassage of electrons to the anode. There is a type II semiconductorinterface between the BFA and the F8. FIG. 5 shows the power efficiencyand the external efficiency of the device against drive voltage. Becauseof the deep HOMO level (5.8 eV) of the F8 relative to the correspondingenergy level in the ITO (4.8 eV) the device needs a high drive voltageand has low power efficiency, even with the intermediate layer of BFA.The power efficiency peaks at about 0.03 lm/W, which is well below whatwould be acceptable in a practical device. And because the powerefficiency is low there is severe heating in the device and its lifetimeis short (a matter of minutes). Even during the measurement period forthe data for FIG. 5 it was found that the device suffered rapid decay,believed to be due to recrystallisation resulting from heating. Thiscauses a shift in the emission spectrum of the device, with lower energyemissions increasing relative to high energy emissions.

FIG. 6 is a band diagram for a second device, in which the emissivelayer is 84% F8 mixed with 16% PFM. A layer of BFA is again included asan intermediate hole transport layer. There are type II semiconductorinterfaces between the BFA and the F8, the BFA and the PFM, and the PFMand the F8.

FIG. 7 shows the power efficiency and the external efficiency of thedevice against drive voltage. Compared to the device of FIGS. 4 and 5the peak power efficiency of this device is increased (0.33 lm/Wcompared to 0.03 lm/W), but the voltage at which the power efficiencypeaks is not reduced. This suggests that in this device holes areinjected into the F8 host polymer (as in the device of FIGS. 4 and 5)and then localised on PFM segments. FIG. 8 shows the emission spectrumfrom the device (line 10) compared to the spectrum of F8 (line 11). FIG.8 indicates that sufficient recombination does occur on the F8 tosuggest that holes are injected into the F8 from the hole transportlayer of BFA, but shows that most of the recombination of holes andelectrons occurs on the PFM rather than the F8. Thus, localised holesform electron-hole pairs with reasonable probability that the electroncan be excited to the PFM region. The peak external efficiency of thisdevice (see FIG. 7) is around 1 Cd/A which is around a factor of 10better than the device of FIGS. 4 and 5.

FIG. 9 is a band diagram for a third device, in which the emissive layeris 84% F8 mixed with 16% TFB. A layer of BFA is again included as anintermediate hole transport layer. There are type II semiconductorinterfaces between at least the BFA and the F8, and the TFB and the F8.FIG. 10 shows the power efficiency and the external efficiency of thedevice against drive voltage. Compared to the device of FIGS. 4 and 5there is a smaller increase in peak external efficiency (to 0.38 Cd/A)and hence a lower peak power efficiency (around 0.15 lm/W). It isbelieved that the reduction in drive voltage is due to the HOMO level ofthe TFB being roughly coincident with the corresponding energy level ofthe hole transport material (BFA)—this facilitates hole injection intothe matrix of the light-emissive layer. FIG. 11 shows the emissionspectrum from the device, which indicates that emission from the device,and therefore radiative recombination within the device, is splitroughly equally between the TFB and the F8 polymers.

FIG. 12 is a band diagram for a fourth device, in which the emissivelayer is 78% F8 mixed with 15% TFB and 7% PFM. A layer of BFA is againincluded as an intermediate hole transport layer. There are type IIsemiconductor interfaces between at least the BFA and the F8, the BFAand the PFM, the TFB and the PFM, the TFB and the F8, and the PFM andthe F8. FIG. 13 shows the power efficiency and the external efficiencyof the device against drive voltage. This device shows remarkableimprovements in performance. The peak external efficiency is around 2.4Cd/A and the peak power efficiency is around 1.05 lm/W. Peak powerefficiency occurs at only around 6.5 V. FIG. 14 shows the emissionspectrum from the device, which indicates that all of the emission isfrom the PFM. The following table compares these results with those forthe devices of FIGS. 4 to 11.

TABLE 2 Voltage at Peak Composition Peak power peak power external ofemissive efficiency efficiency efficiency Emitting layer (lm/W) (V)(Cd/A) material F8 0.03 8.7 0.074 F8 PFM:F8 0.33 9.0 1 PFM + F8 TFB:F80.15 7.0 0.37 TFB + F8 TFB:PFM:F8 1.05 6.5 2.4 PFM

The high efficiency of the device of FIGS. 12 to 14 is especiallysurprising since its emission is from PFM which, as table 1 shows, hasby far the lowest PL efficiency of any of the materials used.

It is believed that in the device of FIGS. 12 to 14 the TFB acts toaccept holes from the hole transport layer into the polymer matrix ofthe emissive layer, the holes then being localised on PFM segments. Thusthe TFB acts to promote injection of holes into the emissive layer. TheLUMO level of the TFB is roughly half way between those of the F8 andthe PFM, so it is believed that the TFB LUMO level also provides anintermediate energy step which enhances the rate of transfer ofelectrons to the PFM when the device is under bias. Also, PFM has aslightly lower optical gap than F8 or TFB, making it energeticallyfavourable for the excitons to form on the PFM regions.

The device of FIGS. 12 to 14 could be adapted by substituting PFMO forthe PFM. PFMO has a PL efficiency of 40% (see table 1) and the resultingdevice has a power efficiency of up to 1.5 lm/W, with emission beingfrom the PFMO. Since PFMO has the same optical gap as F8 and TFB (seetable 1) this result suggests that Förster transfer is not the dominantmechanism by which the exciton is confined to the PFMO polymer, althoughit can be envisaged that it could enhance efficiency. Instead, it isbelieved that under bias the energy line-up between the hole transportlayer and the TFB promotes hole injection into the matrix of theemissive layer. This is followed by transfer to the lower energy HOMOlevel of the PFMO. However, when TFB is present there is also an energystep in the LUMO levels roughly half way between those of the PFMO andthe F8. Thus, when the device is biased the TFB promotes electrontransfer on to PFMO polymer segments. In practice there is likely to bea distortion of the energy levels near the polymer interfaces, unlikethe simple representation in FIG. 12.

This explanation is supported by the results of changing the cathodefrom LiAl to CaAl. CaAl has a higher workfunction than LiAl. If electrontransport through the matrix of the emissive layer were via F8, followedby excitation to the PFM or the PFMO then the use of a higherworkfunction material should not affect the efficiency, because theworkfunction is still close to the LUMO level of F8. However, ifelectrons were injected from the LiAl cathode and transferred to the PFMpredominantly via the TFB then power efficiency would be expected tofall, because of the higher drive voltage that would be required toovercome the barrier to electron injection between CaAl and TFB. Theapplicant has observed no significant difference in performance betweendevices with LiAl or CaAl cathodes.

FIG. 15 is a band diagram for a green emitting device in which theemissive layer is another two-component polymer mixture. In this devicethe emissive layer is 95% F8 mixed with 5% F8BT. The F8BT dopant forms atype I semiconductor interface with the host F8, but both form type IIsemiconductor interfaces with the BFA. FIG. 16 shows the powerefficiency and the external efficiency of the device against drivevoltage. The power efficiency is around 2.0 to 2.5 lm/W. FIG. 17 showsthe power efficiency and the external efficiency against drive voltagefor a device similar to that of FIGS. 15 and 16 but in which theemissive layer is a three-component mixture: of first F8 mixed with F8BTin the ratio 19:1, and then that mixture mixed with TFB in the ratio 4:1(i.e. (F8:F8BT [0.95:0.05]):TFB [0.75:0.25]). FIG. 18 is a band diagramfor such a device. FIG. 19 shows the power efficiency and the externalefficiency against drive voltage for a device similar to that of FIGS.15 and 16 but in which the emissive layer is mixed as (F8:F8BT[0.95:0.05]):TFB[0.5:0.5]. The results for these three devices aresummarised in the following table.

TABLE 3 Amount of Voltage at Peak TFB in Peak power peak power externalVoltage at emissive efficiency efficiency efficiency peak external layer(%) (lm/W) (V) (Cd/A) efficiency (V) 0 2.4 6.8 5.2 Approx. 8.5 25 6 3.88 4.5 50 6.7 3.5 7.8 3.8

As the amount of TFB reaches between 50 and 60% the peak efficiencydecreases. There is also an increase in external efficiency when TFB isadded, principally over the range from 0 to 20% TFB. This is valuablefor practical applications.

Efficiencies of greater than 20 lm/W may be achieved using these5F8BT:TFB 80:20 structures with PEDOT:PSS as the hole-transport layer.(See FIG. 22).

It is believed that, as with the three-component blue-emissive mixturesdescribed above, the TFB promotes hole injection into the polymer matrixof the emissive layer, allowing exciplexes to form. In thegreen-emissive devices the exciplexes have a relatively high probabilityof forming excitons on the F8BT polymer because of the higher internalfield and because by doing so they can reduce energy by Förster transfer(F8BT having the narrowest optical gap in the matrix). This leads to animprovement in the external efficiency and the power efficiency.

The efficiency of this device may be further improved by including inthe emissive layer one or more other polymers whose HOMO levels arebetween those of the TFB and the F8 (e.g. around 5.5 eV). This shouldpromote excitation of holes from the TFB to the emissive material, andat higher bias fields promote hole injection into the matrix of theemissive layer itself.

It should be noted that including PFM in the green-emissive blendsubstantially reduces device efficiency. This is believed to be due toits relatively shallow HOMO level, which acts as a deep hole trap,rather than as an intermediary, and thus inhibits formation of excitonson the F8BT polymer.

FIGS. 20 to 27 illustrate some other embodiments of the principlesdiscussed above.

In FIG. 20 the emissive layer is a mixture of F8BT, F8 and PPV. TheF8BT:F8 blend is coated over the PPV and acts as an electron transportlayer allowing electron transport between the cathode and the conductionband in the PPV. The addition of the F8BT and F8 to the PPV allows theuse of a cathode that is more stable than the usual AlLi cathodes, eventhough it has a higher workfunction. In FIG. 18 the cathode is Mg, whoseworkfunction is 3.5 eV. An alternative is Al, whose workfunction is 4.2eV. A further improvement could be made by including in the blend athird polymer with a LUMO level between those of F8BT (3.4 eV) and F8(2.8 eV).

In FIG. 21 the emissive layer is a mixture of poly(paraphenylene)(“PPP”), TFB and F8 with a CaAl cathode. The principles described abovein relation to improving hole injection are applied in this device toimprove electron injection.

FIG. 22 illustrates the effect of the addition of TFB to afluorene-based emitter system. The upper panel of FIG. 22 shows theluminance and luminous intensity at a range of voltages for a device inwhich the emissive layer is 5F8BT. The lower panel of FIG. 22 showsequivalent data for a device in which the emissive layer is 5F8BT with20% TFB. Both devices have a PEDOT:PSS hole transport layer. The resultsshow that the addition of TFB to the emissive layer improves peakefficiency from around 3.5 lm/W to around 20 lm/W. It has been foundthat PEDOT:PSS is superior to BFA as a hole transport layer in suchdevices, especially when “electronic grade” PEDOT:PSS is used.

FIGS. 23 to 25 show data for devices in which the emissive component isnot fluorene-based. FIG. 23 shows a set of plots of luminance (Cd/m²),current density (mA/cm²), luminous efficiency (lm/W) and externalefficiency (Cd/A) at a range of voltages for an 8 pixel device in whichthe emissive layer is the soluble PPV emitter Bis-DMOS PPV (bis dimethyloctyl silyl poly phenylene vinylene, with two side units of the formulabeing SiMe₂C₈H₁₇ on the phenyl ring, see FIG. 27). The emissive layerwas spun on to hole transport layers of PEDOT:PSS. FIG. 24 showsequivalent data for devices in which the emissive layer is Bis-DMOS PPVwith 25% TFB. The addition of the TFB was found to improve turn-onvoltage from around 3.5 to around 2.5V and to increase the efficiency toaround 2.0 lm/W. FIG. 25 shows plots of the characteristics of a seriesof devices in which the emissive layer is formed from another solublePPV (PPV A of table 1) with additions of 0%, 2%, 10% or 20% TFB. Theupper panel of FIG. 25 plots the current density of such devices againstvoltage. The middle panel of FIG. 25 plots the luminance of such devicesagainst voltage. The lower panel of FIG. 25 shows the luminance of suchdevices at a constant voltage of 5.6V. The luminance and current densitywas found to increase with increasing TFB content. At 5.6V the additionof 20% TFB was found to increase luminance by around 400% over a devicewith 0% TFB.

FIG. 26 shows a set of plots of luminance (Cd/m²), current density(mA/cm²), luminous efficiency (lm/W) and external efficiency (Cd/A) at arange of voltages for a 4 pixel device in which the emissive layer wasformed from a mixture of 70% 5F8BT and 30% of a polyethylene-basedpolymer with TPD-based side-chains. The maximum efficiency was found tobe 8 lm/W whereas similar devices in which the emissive layer was of5F8BT alone were found to have maximum efficiencies of 2 lm/W. FIG. 29shows a cyclic voltammetry oxidation sweep of the polyethylene-basedpolymer with TPD-based side-chains, showing that its HOMO level is inthe region of 5.25 eV, i.e. between that of the emissive component andthat of the hole-injecting layer.

In some cases it may be expected that a degree of surface phaseseparation will occur, with one or more of the components of thelight-emissive layer increasing in concentration near a surface of theemissive layer due to the interactions of the material(s) of theemissive layer with the adjacent surfaces. Methods that could be usedfor detecting such separation include an atomic force microscope intapping mode, and other similar techniques.

FIG. 28 shows plots of luminance at a range of voltages for a pair ofdevices in which the emissive layer is formed from a blend of 5F8BT:TFB(4:1) deposited over a hole transport layer of PEDOT:PSS. In one device(line A) the emissive blend was deposited directly on to the PEDOT:PSS.In the other device (line B) a thin intermediate layer of TFB wasdeposited directly on to the PEDOT:PSS and the emissive blend was thendeposited over that layer. FIG. 28 shows that the device having theintermediate layer of TFB was found to show significantly increasedluminance at fixed voltage. This is believed to be due to the formationof a concentration gradient of the TFB component in the blend as aresult of greater attraction of that component to the intermediate TFBlayer than of the 5F8BT. Since the TFB is capable of acting as a holetransport/capture component in the blend it may be expected to beadvantageous for that component to be more concentrated towards thehole-injecting side of the emissive layer, i.e. nearer to the interfacewith the PEDOT:PSS. Similar results could be expected from other methodsof encouraging such a concentration gradient and from the creation of ananalogous concentration gradient of any electron transport/capturecomponent towards the electron-injecting side of the emissive layer.

As an indication of the effectiveness of the intermediate layer of TFB,table 4 shows the peak luminous efficiencies found for four device typesunder test:

TABLE 4 Composition of Intermediate Peak luminous emissive layer layerof TFB? efficiency (lm/W) 5F8BT No 6 5F8BT Yes 8 5F8BT:TFB (4:1) No 95F8BT:TFB (4:1) Yes 12

A multi-component layer of the types described above is preferablydeposited as a single layer. To achieve this the components that are tomake up the layer are preferably combined together before or duringdeposition. Such combination suitably involves forming a material thatincludes the components, for example by physically mixing the componentstogether or by forming a chemical mixture by (for instance) formingmolecules that incorporate one or more of the components.

One example of a material in which the components are combinedchemically is a terpolymer of F8, F8BT and TFB, for example (5F8BT):TFB[80:20]. Such a material may be formed by the following method. A 500 mlr.b. flask fitted with a reflux condenser (connected to a nitrogen line)and a mechanical stirrer is charged with9,9-dioctylfluorene-2,7-diboronic acid 1,2 ethyleneglycol diester (4.773g, 9.0 mmol), 2,7-dibromobenzothiadiazole (0.1504 g, 0.51 mmol),2,7-dibromo-9,9-dioctylfluorene (3.584 g, 6.54 mmol),N,N′-di-(4-bromo)phenyl-4-(sec-butyl)aniline (0.8923 g, 1.945 mmol), 90ml of toluene, tetrakis(triphenylphosphine) palladium (31.2 mg) and 20ml of 2 molar sodium carbonate solution. The mixture is stirred rapidlyand heated at 115° C. (oil bath temperature) for up to 18 hours. Afurther 100 ml of toluene is added along with bromobenzene (1 ml), themixture is then allowed to stir at temperature (115° C. oil bathtemperature) for a further three hours. Phenyl boronic acid (1.5 g) isthen added and the mixture is stirred at temperature for one hour andthen allowed to cool to room temperature. The reaction mixture is thenpoured slowly into 4 l of methanol to precipitate the polymer. Thepolymer may then be deposited (for instance from solution) to form anemissive layer of a light-emissive device. In general, one route forformation of suitable terpolymers may be to partially react monomersproviding two of the components and then to add monomers providing athird component and allow the polymerisation to continue. The copolymerscould be of any appropriate form, for example random, block or branchedcopolymers.

Either of the charge transporting components plus the light emittingcomponent, or both charge transporting components plus the lightemitting component, may be combined in a single molecule. Where twocomponents are combined, a copolymer is formed, while the combination ofall three components may be achieved in a terpolymer. In a physicalmixture of components, there might not be intimate mixing of thecomponents due to phase separation, which might produce inefficientcharge transport from either or both of the charge transportingcomponents to the light emitting component. In a molecule that combinesthe charge transporting and light emitting components, these units arechemically connected and so charge transport may be expected to befacilitated compared to the physical mixture case. Molecules containingboth hole transporting and light emitting components may be physicallycombined with an electron transporting species to produce an efficientlight-emitting layer. Alternatively, molecules containing both electrontransporting and light emitting components may be physically combinedwith a hole transporting species. If one of the electrodes is especiallyeffective then an efficient LED structure may be produced by omittingthe charge transporting component corresponding to that electrode andusing for the emissive layer a two component molecule, comprising asuitable opposite charge transporting component and a light emittingcomponent.

Two components of the emissive layer could be mixed chemically into asingle molecule, and that material mixed physically with a thirdcomponent provided by a second molecule.

The devices described above can be fabricated in the conventional way,by deposition of the polymer layers from solution on to acommercially-available glass/ITO substrate, followed by evaporativedeposition of the cathode. As an example, the fabrication of the deviceof FIGS. 4 and 5 will be described in detail. First, on to acommercially-available glass/ITO substrate the BFA is deposited byspin-coating from a 0.8% concentration solution withN,N-Dimethylformamide (“DMF”) as the solvent. The polymer layer is thendried by heating the sample to 80° C. under nitrogen gas for 30 minutes.Then the emissive layer is spin-coated from a 1.6% concentrationsolution with mixed xylene as the solvent. Targets for the thicknessesof the BFA layer and the emissive layer are 10 to 80 nm and 60 to 200 nmrespectively; typical thicknesses are 65 nm and 95 nm respectively.Finally the cathode is deposited by evaporation to form a 20 nm thicklayer of Li, followed by a 200 nm layer of Al. In an inert atmospheresuch as nitrogen the device is encapsulated by sealing within aglass/epoxy encapsulant.

Alternative materials could be used in devices that embody theprinciples described above. For example, alternative materials for theanode include tin oxide (“TO”) and flurinated TO; alternative materialsfor the hole transport layer include PEDOT:PSS and poly-aniline;alternative materials for the emissive layer include PPV and MEH-PPV;and alternative materials for the cathode include Li, Ba, Mg, Ca, Ce,Cs, Eu, Rb, K, Sm, Na, Sm, Sr, Tb and Yb; an alloy of two or more ofsuch metals; a halide (e.g. fluoride), carbide, oxide or nitride of oneor more of such metals (e.g. CsF, LiF); and an alloy of one or more ofsuch metals together with one or more metals such as Al, Zr, Si, Sb, Sn,Zn, Mn, Ti, Cu, Co, W, Pb, In or Ag (e.g. LiAl). The cathode could betransparent. The hole transport layer could be omitted, or there couldbe more than one hole transport layer. There could be one or moreelectron transport layers between the cathode and the emissive layer,which would act to facilitate transfer of electrons from the cathode tothe emissive layer and/or to block passage of holes to the cathode.

The hole transport layer could be of a mixture of materials. Forinstance, it could be of BFA blended with a PFM-like polymer but withsolution solvency as for the BFA adjusted by the inclusion of carboxylicacid groups. Any electron transport layer could also comprise a mixtureof materials, such as F8 and F8BT.

In addition to providing highly efficient blue and green emissivedevices, the principles described above could be used for longerwavelength (e.g. red) emissive devices. There are polymers that arehighly desirable for such devices because they have deep LUMO levels,which would allow higher workfunction and more stable metals to be usedfor the cathode, and high PL efficiency. However, those polymers havecorrespondingly deep HOMO levels. The principles described above wouldallow those polymers to be used despite their deep HOMO levels.

When fabricating devices with multi-component emissive layers thecomponents can suitably be mixed together before deposition. However, tooptimise the electronic effects of the components so as to exploit theprinciples described above it is preferred that the components in theemissive layer are partially or fully phase-separated, so that withinthe emissive layer there are dispersed regions of each component (seeU.S. Pat. No. 5,760,791, the contents of which are incorporated hereinby reference). Where movement of charge carriers from one component toanother is desired, the structure of the mixed layer (especially theproportions of these components and the sizes of the regions of each)should preferably provide for adequate interfaces between thosecomponents. The regions of each component could be dispersed evenlythrough the layer or it may be advantageous for one or more of thecomponents to be concentrated near one or other of the interfaces of thelight-emissive layer with the charge carrier injection layers. It may befound desirable for there to be a greater concentration of ahole-injecting material at the side of the emissive layer nearer theanode (and less at the side nearer the cathode). It may be founddesirable for there to be a greater concentration of anelectron-injecting material at the side of the emissive layer nearer thecathode (and less at the side nearer the anode).

In order to influence the dispersion characteristics of the mixed layer(including the profile of the various components through the layer) anumber of techniques may be used. The surface interactions between oneor more of the components and other layers of the device (especially thelayer on to which the emissive layer is deposited) may be employed topromote some degree of phase separation in the emissive layer.Preferably the deposition conditions may be arranged so that theappropriate one of the hole- or electron-transport materials ispreferentially attracted to or repelled from the layer on which theemissive layer is deposited. Thus if the emissive layer is deposited ona hole-injecting anode layer then preferably the hole-injecting materialis preferentially attracted to that surface and/or theelectron-injecting material is preferentially repelled from the surface.The opposite preferably applies if the emissive layer is deposited on anelectron-injecting cathode layer. The material of the surface on towhich the emissive layer is to be deposited may be treated or selectedto give rise to such preferential effects. Where two or more of thecomponents of the emissive layer are combined in the same molecule theway in which they are combined may be used to promote a desired state ofphase separation (or segregation) or uniformity. For example, thecombined molecules could be synthesised as suitable random, block orbranched copolymers to achieve the desired effect. Other factorsrelevant for phase separation include the solvent used (e.g. chloroformor xylene), the solvent vapour pressure, the molecular weight, componentsurface energies (relative to each other and/or to the free surface onto which the emissive layer is deposited), poly dispersity, componentvolume fractions, structural features etc.

The present invention may include any feature or combination of featuresdisclosed herein either implicitly or explicitly or any generalisationthereof irrespective of whether it relates to the presently claimedinvention. In view of the foregoing description it will be evident to aperson skilled in the art that various modifications may be made withinthe scope of the invention.

The invention claimed is:
 1. An electroluminescent device comprising: afirst charge carrier injecting layer for injecting positive chargecarriers; a second charge carrier injecting layer for injecting negativecharge carriers; an organic light-emissive layer located between thecharge carrier injecting layers; and a plurality of positive chargecarrier transport layers located between the light emissive layer andthe positive charge carrier injecting layer; wherein a luminescent typeII heterojunction is formed between one of the positive charge carriertransport layers and the light-emissive layer; wherein the plurality ofpositive charge carrier transport layers provide a series ofintermediate energy steps between the light-emissive layer and thepositive charge carrier injection layer.
 2. An electroluminescent deviceaccording to claim 1, wherein one or more of the positive charge carriertransport layers are organic.
 3. An electroluminescent devicecomprising: a first charge carrier injecting layer for injectingpositive charge carriers; a second charge carrier injecting layer forinjecting negative charge carriers; an organic light-emissive layerlocated between the charge carrier injecting layers; and a plurality ofpositive charge carrier transport layers located between the lightemissive layer and the positive charge carrier injecting layer; whereina luminescent type II heterojunction is formed between one of thepositive charge carrier transport layers and the light-emissive layer;wherein a further organic charge transporting layer is located betweenthe light-emissive layer and the negative charge carrier injectinglayers; and wherein the plurality of positive charge carrier transportlayers provide a series of intermediate energy steps between thelight-emissive layer and the positive charge carrier injection layer. 4.An electroluminescent device according to claim 1, comprising: a firstcharge carrier injecting layer for injecting positive charge carriers; asecond charge carrier injecting layer for injecting negative chargecarriers; an organic light-emissive layer located between the chargecarrier injecting layers; and a plurality of positive charge carriertransport layers located between the light emissive layer and thepositive charge carrier injecting layer; wherein a luminescent type IIheterojunction is formed between one of the positive charge carriertransport layers and the light-emissive layer; wherein the organiclight-emissive layer comprises a light-emissive component and a chargetransporting component; and wherein the plurality of positive chargecarrier transport layers provide a series of intermediate energy stepsbetween the light-emissive layer and the positive charge carrierinjection layer.
 5. An electroluminescent device according to claim 4,wherein the charge transporting component is for transporting negativecharge carriers.
 6. An electroluminescent device according to claim 4,wherein the organic light-emissive component and the charge transportingcomponent are provided as components of a single molecule.
 7. Anelectroluminescent device according to claim 4, wherein the organiclight-emissive component and the charge transporting component form atype II semiconductor interface.
 8. A method for forming anelectroluminescent device, comprising: depositing a first charge carrierinjecting layer for injecting positive charge carriers; depositing anorganic light-emissive layer over the first charge carrier injectinglayer; and depositing a second charge carrier injecting layer forinjecting negative charge carriers over the organic light-emissivelayer, wherein the method comprises a further step of depositing aplurality of positive charge transport layers located between thelight-emissive layer and the positive charge carrier injecting layerforming a luminescent type II heterojunction between one of the positivecharge carrier transport layers and the light-emissive layer; andwherein the plurality of positive charge carrier transport layersprovide a series of intermediate energy steps between the light-emissivelayer and the positive charge carrier injection layer.
 9. A methodaccording to claim 8, wherein the organic light-emissive layer and oneor more of the positive charge carrier transport layers are depositedfrom solution.
 10. A method according to claim 9, wherein the organiclight-emissive layer and one or more of the positive charge carriertransport layers are deposited together from solution and phase separateto form the organic light-emissive layer and the one or more positivecharge carrier transport layers.
 11. A method according to claim 9,wherein the organic light-emissive layer and one or more of the positivecharge carrier transport layers are deposited in separate depositionsteps.
 12. A method according to claim 8, wherein one or more of thepositive charge carrier transport layers are organic.
 13. A method forforming an electroluminescent device, comprising: depositing a firstcharge carrier injecting layer for injecting positive charge carriers;depositing an organic light-emissive layer over the first charge carrierinjecting layer; and depositing a second charge carrier injecting layerfor injecting negative charge carriers over the organic light-emissivelayer; wherein the method comprises a further step of depositing aplurality of positive charge transport layers located between thelight-emissive layer and the positive charge carrier injecting layerforming a luminescent type II heterojunction between one of the positivecharge carrier transport layers and the light-emissive layer; wherein afurther organic charge transporting layer is disposed between the lightemissive layer and the negative charge carrier injecting layer; andwherein the plurality of positive charge carrier transport layersprovide a series of intermediate energy steps between the light-emissivelayer and the positive charge carrier injection layer.
 14. A methodaccording to claim 8, for forming an electroluminescent device,comprising: depositing a first charge carrier injecting layer forinjecting positive charge carriers; depositing an organic light-emissivelayer over the first charge carrier injecting layer; and depositing asecond charge carrier injecting layer for injecting negative chargecarriers over the organic light-emissive layer, wherein the methodcomprises a further step of depositing a plurality of positive chargetransport layers located between the light-emissive layer and thepositive charge carrier injecting layer forming a luminescent type IIheterojunction between one of the positive charge carrier transportlayers and the light-emissive layer, and wherein the organiclight-emissive layer comprises a light-emissive component and a chargetransporting component.
 15. A method according to claim 14, wherein thecharge transporting component is for transporting negative chargecarriers.
 16. A method according to claim 14, wherein the organiclight-emissive component and the charge transporting component areprovided as components of a single molecule.
 17. A method according toclaim 14, wherein the organic light-emissive component and the chargetransporting component form a type II semiconductor interface.